The present invention relates to certain nonionic oligomeric surfactants, in particular such surfactants based on vinyl systems having an amido function, especially acrylamide, methacrylamide or a derivative of either of these, and to their use as coating aids, especially for use in the manufacture of aqueous coated photographic products.
Coating aids facilitate the coating process. For example, coating aids are used in the manufacture of photographic products, such as photographic films and the like, for controlling coating uniformity after the application of the coated layers onto a product support (such as film or paper), for the period the coating remains fluid. After the coating point, the coated layers pass through a chill box. During their passage through this section the coated layers set to a rigid gel. Hence, the coated layers remain fluid from their application point to a position somewhere inside the chill box section. This period of fluidity amounts to a period of seconds at current coating speeds. During this period, the coating is subject to the impact of disturbances or perturbations which result from vibrations or air currents, present as part of the mechanics of the coating process.
Amongst commonly known coating aids, saponin is unusual insofar as it provides control of post-coating flow over a wide range of concentrations. However, in the multilayer coating of colour photographic products, it is typical for the lower layers to possess dynamic surface tensions  less than 35 mN.mxe2x88x921 due to the surface activity of the dispersing aids used therein, e.g. dispersants for colour couplers such as Alkanol XC(trademark) (DuPont) which leads to surface tension values in the range 32-35 mN.mxe2x88x921. To remain spread, the uppermost layer needs to possess a lower surface tension than any of the underlying layers. Unfortunately, saponin is not very effective at lowering surface tension insofar as its limiting surface tension in aqueous gelatin solution is about 38 mN.mxe2x88x921. Hence it is clear, that in such cases, saponin cannot be used in isolation in the uppermost layer to improve coating uniformity, for if it is used, coating quality becomes unacceptable due to retraction of the top layer over the underlying ones. Furthermore, this deficiency cannot be overcome by using saponin in combination with a more surface-active regular coating aid, like Aerosol OT(trademark) (Cytec), as such materials cause substantive increases in the susceptibility of the coating to disturbances due to vibrations or air currents. Therefore, the ability to manipulate surface tension to values  less than 35 mN.mxe2x88x921 whilst maintaining good coating uniformity would be of substantial benefit in simultaneous multi-layer coating where the relative dynamic surface tensions between layers is critical as described above.
It is well known in the art that gelatin coatings show a tendency to become electrostatically charged on separation from surfaces that they come into contact with, such as transport rollers. The nature of the electrostatic charging depends on the relative triboelectric properties of the two contacting surfaces and the conditions prevailing at the time of the charging, such as: speed of contact and separation processes; pressure of contact; temperature; relative humidity; and so on. It is also well known in the art that gelatin-based coatings containing fluorocarbon surfactants often tend to charge electrically in the opposite direction to those containing hydrocarbon-based surfactants. Hence it is common practice to coat mixtures of fluorocarbon and hydrocarbon surfactants in the uppermost layers of photographic products in order to minimise the electrostatic charging from a triboelectric point of view.
The present invention, in one of its aspects, provides hydrophobically capped oligomeric acrylamides or pyrrolidones, in particular the compounds of the general formula 
in which
R, R1 and R2 are hydrophobic groups, in particular are each independently selected from aliphatic hydrocarbyl, aryl-(aliphatic hydrocarbyl) and (aliphatic hydrocarbyl)-aryl groups, which groups may be unsubstituted, partially fluorinated or fully fluorinated,
L is a sulphur atom or a linking group that contains a sulphur atom that is attached to the T group, and
T is a hydrophilic oligomeric group obtained by the oligomerisation of one or more vinyl monomers having an anido function.
The present invention also provides, in a further aspect thereof, a coating composition that comprises a liquid medium that contains a hydrophilic colloid and a surfactant, wherein the said surfactant is a compound of the present invention as defined above.
The present invention, in another aspect, provides a method of coating a substrate, which comprises applying to the substrate a coating composition of the present invention. The invention also extends to the coated substrates so produced, especially such coated substrates that may be used in or as photographic products, such as photographic films.
The present invention, in yet another aspect thereof, provides the use of a compound of the present invention, as defined above, as a coating aid in a liquid coating composition.
The preferred compounds of the present invention are defined by either of the following formulae: 
The number of hydrophobic groups, R or (R1+R2), depends on the linking group L. The hydrophobic group or groups each comprise an aliphatic hydrocarbyl, aryl-(aliphatic hydrocarbyl) or (aliphatic hydrocarbyl)-aryl group, wherein, in each case, the aliphatic hydrocarbyl group may be saturated or unsaturated and may be straight or branched. These groups may be non-fluorinated, completely fluorinated or partially fluorinated. Preferred aliphatic hydrocarbyl groups are alkyl and alkenyl groups and preferred aryl moieties are phenyl and naphthyl groups. Typically, each of the groups R, R1 and R2 contains from 8 to 21 carbon atoms. In certain preferred embodiments R1 and R2 together have a total of from 8 to 21 carbon atoms. R1 and R2 may be the same or different.
The linking entity L is either a sulphur atom (xe2x80x94Sxe2x80x94) or a linking group that contains a sulphur atom through which it is attached to T, in other words a linking group that is linked to the or each hydrophobic group by a simple chemical bond and to the oligomeric moiety T by a thio-link (xe2x80x94Sxe2x80x94).
Certain preferred sub-groups of the present compounds are represented by the following general formulae, in which formulae (3), (4) and (5) contain typical linking groups for compounds with one hydrophobic group and formulae (6), (7) and (8) contain typical linking groups for compounds with two hydrophobic groups:
Rxe2x80x94Sxe2x80x94Txe2x80x83xe2x80x83(3)
Rxe2x80x94Oxe2x80x94OCxe2x80x94CH2xe2x80x94Sxe2x80x94Txe2x80x83xe2x80x83(4)
Rxe2x80x94NHxe2x80x94OCxe2x80x94CH2xe2x80x94Sxe2x80x94Txe2x80x83xe2x80x83(5)

In the above formulae (4), (5), (7) and (8), other simple chemical groups, in place of the ester or amide link, may also come into consideration.
The oligomeric group T is preferably based on the oligomerisation of vinyl monomers with an amido function, the vinyl part providing a route to oligomerisation and the amido part providing a nonionic polar group to constitute the hydrophilic functional group (after oligomerisation). The oligomeric group T can be constituted from a single monomer source or a mixture of two or more monomers, provided that the resultant oligomeric chain is sufficiently hydrophilic to render the resulting surface-active material soluble in water or other medium. A mixture of monomers may give rise to a group T that is a random co-oligomer. Preferred monomers used to make the oligomeric chain T are acrylamide and derivatives of acrylamide, although methacrylamide and derivatives of methacrylamide can be used too. The monomer 2-vinylpyrrolidone may also be used, although it is less favoured owing to the possibility of adverse photographic effects sometimes found with polyvinyl pyrrolidone (PVP) materials.
Certain suitable monomers can be represented by the two general formulae 
wherein formula (9) represents acrylamide, methacrylamide or a derivative of either of hese, and formula (10) represents 2-vinylpyrrolidone (also abbreviated to Py hereinafter). In formula (9), R3 is H or a C1, C2 or C3 alkyl group, preferably H or CH3 (which leads to an acrylamide or methacrylamide-based monomer, respectively) and Y and Z, which may be the same or different, are each H, C1-C3 alkyl or C1-C4 alkyl substituted with one or more hydroxyl groups, preferably H, CH3, C2H5 or C(CH2OH)3. Alternatively, Y and Z, together with the N atom to which they are shown attached, may form a heterocyclic ring, in particular a 5- or 6- membered ring, e.g. a pyrrolidinyl or piperidinyl ring.
In certain preferred embodiments, the compounds of the formulae (1) and (2) have an oligomeric group (T) that contains from 2 to 100, more preferably from 3 to 50, and typically from 5 to 40, monomeric units. The degree of oligomerisation is generally selected so as to provide sufficient hydrophilicity to render the resulting compound soluble in the aqueous medium.
The hydrophobically capped oligomeric acrylamide or pyrrolidone dispersants useful in the present invention may be prepared by processes similar to those described in the preparative Examples hereinafter and in Pavia et al, Makromol. Chem. (1992), 193 (9), 2505-2517.
Exemplary compounds of the present invention are described in the following Table A. Each compound therein is allocated a reference number (I-1, I-2, etc.) by which it is identified in the Test Examples hereinafter.
As indicated above, the compounds of the present invention are surface-active agents that find use as coating aids. In certain preferred embodiments, a coating composition comprises an aqueous solution of a hydrophilic colloid and a compound of the present invention present as a coating aid. In certain other preferred embodiments, a product, e.g. a photographic material, comprises one or more layers coated on a support, each such layer comprising or consisting of a hydrophilic colloid composition; such a product may be made by coating the support with one or more layers, each comprising or consisting of a hydrophilic colloid coating composition, and drying the layer or layers, wherein at least one of the layers is a coating composition according to this invention. The drying of the layer(s) may be preceded by the chill setting of the coated colloid layers. In certain particularly preferred embodiments, a photographic material comprises a support and at least one hydrophilic colloid layer and at least one light-sensitive silver halide emulsion layer, at least one hydrophilic colloid layer comprising a compound according to the present invention. It is generally preferred to coat the aqueous hydrophilic colloid layers onto a support simultaneously. The compounds of the invention are particularly useful as coating aids when incorporated in the outermost or topmost layer, e.g. a protective overcoat, of a photographic material; however, the use of the compounds in underlying layers is not precluded.
The types of layers that may be included in a photographic material and suitable components for use in photographic products are known in the art and include those which are mentioned, for example, in Research Disclosure, December 1978, Item 17643, and in U.S. Pat. No. 5,366,857 and U.S. Pat. No. 5,474,889, as are procedures whereby the coating and drying of the hydrophilic colloid layers may be carried out. Components for use in photographic products are also disclosed in Research Disclosure, December 1989, Item 308119, published by Kenneth Mason Publications Ltd. The disclosure in each of the aforesaid publications is incorporated herein by reference.
The preferred hydrophilic colloid is gelatin, e.g. alkali-treated gelatin, acid-treated gelatin, deionized gelatin or a gelatin derivative such as acetylated or phthalated gelatin. Other hydrophilic colloids, however, may come into consideration, for example naturally occurring substances such as proteins, protein derivatives, cellulose derivatives (e.g. cellulose esters), polysaccharides (e.g. dextran, gum arabic, zein, casein or pectin), collagen derivatives, agar-agar, arrowroot or albumin, or synthetic hydrophilic colloids such as polyvinyl alcohol, acrylamide polymers, maleic acid copolymers, acrylic acid copolymers, methacrylic acid copolymers or polyalkylene oxides.
The present compounds will usually be employed in the coating compositions at a concentration of from 0.01 to 1.5 percent by weight, typically from 0.02 to 1 percent by weight.
The use of coating aids, to control uniformity after application and prior to setting in the chill box, has been mentioned. The materials of the invention offer the advantage that they provide better uniformity in that context than most simple coating aids. Furthermore, they offer the additional advantage that the improved coating uniformity is attained independently of their concentration over a wide range. In contrast, coating uniformity becomes progressively worse with increasing concentration with most simple coating aids.
The desirability of producing surface tension values of less than 35 mN.mxe2x88x921 whilst maintaining good coating uniformity has also been mentioned. In this context, the materials of the invention possess a major advantage over saponin, the advantage being that their chemical structure can be easily modified to provide more effective lowering of surface tension. In contrast, saponin, being a natural product (source: extracted from soapbark), is not capable of significant variation. An example of the way in which the materials of the invention can be modified in a beneficial way is to change the hydrophobic tail portion of their structure to one with a perfluorocarbon tip; this provides the capability for surface tension reduction to values  less than 20 mN.mxe2x88x921, i.e. significantly  less than 35 mN.mxe2x88x921. The dynamics of adsorption is also dependent on chemical structure and hence can be modified accordingly and systematically; this is plainly of importance to a dynamic process such as coating.
Mention was also made of using a mixture of fluorocarbon surfactant and hydrocarbon surfactant to reduce tribo-electric charging. This is attainable with the materials of the invention since hydrocarbon and fluorocarbon versions can be used as mixtures without destroying their ability to promote good coating uniformity. With saponin there is no parallel situation, owing to the absence of a fluorinated version of saponin: if saponin is used in mixtures with other simple fluorosurfactants the latter will destroy its above-described coating uniformity properties. In fact, one good option for coating per se is the combination of a fluorocarbon embodiment of the invention with saponin as a hydrocarbon-based co-surfactant.